Coiled steel tubing finds a number of uses in oil well operations. For example, it is used in running wireline cable down hole with well tools, such as logging tools and perforating tools. Such tubing is also used in the workover of wells, to deliver various chemicals downhole and perform other functions.
Steel coiled tubing is capable of being spooled because the steel used in the product exhibits high ductility (i.e. the ability to plastically deform without failure). The spooling operation is commonly conducted while the tube is under high internal pressure which introduces combined load effects. Unfortunately, repeated spooling and use causes fatigue damage and the steel coiled tubing can suddenly fracture and fail. The hazards of the operation and the risk to personal and the high economic cost of failure in down time to conduct fishing operations forces the product to be retired before any expected failure after a relatively few number of trips into a well. The cross section of steel tubing expands during repeated use resulting in reduced wall thickness and higher bending strains with associated reduction in the pressure carrying capability. Steel coiled tubing is limited as to internal pressures up to about 5000 psi. Higher pressures significantly reduce the integrity of the tubing so that it will not sustain continuous flexing and thus severely limit its life, even to a single field application.
It is therefore desirable to provide a non-steel coil tubing which is capable of being deployed and spooled under borehole conditions, which does not suffer from the limitations of steel tubing and is highly resistant to chemicals.
Additionally, present steel coiled tubing logging operations sometimes utilize a wireline cable inserted within the bore of the coiled tubing to transmit data, or when real time data is not required, a logging tool capable of collecting and storing data downhole. When real time data is required, a dedicated reel of coiled tubing is used with the wireline permanently installed in the tubing. This takes up substantial cross-sectional space within the tubing and thus renders the coiled tubing unsuitable for other operations requiring a flow path or open bore through the tubing. Fluids are sometimes transported from the surface to a downhole location through the tubing to provide means for treating formations or for operating a mud motor to drill through the formations. In addition, it may be desirable to pump devices through the coiled tubing bore to a downhole location for various operations. Therefore, an open bore within the coiled tubing is essential for many operations and for this reason it is desirable not to have electrical conductors or the like positioned within the open bore portion of the tubing.
External pressures on the coiled tubing are also a major load condition and can be in excess of 2500 psi.
The Prior Art
U.S. Pat. No. 3,554,284 to Nystrom teaches the use of a logging cable in which two inner layers of fibers are wound at .+-.18.degree. and two outer layers are wound at .+-.35.degree..
U.S. Pat. No. 4,255,820 to Rothermel et al. discloses a prosthetic ligament formed with a densely woven cylindrical core that provides the axial stiffness for the prosthesis.
U.S. Pat. No. 4,530,379 to Policelli teaches a composite fiber tubing with a transition to a metallic connector. The fibers may be graphite, carbon, aramid or glass. These fibers, in one embodiment, are alternatively laid in .+-.15.degree. orientations to the longitudinal axis. In the FIG. 4 embodiment, "a wider choice of axial angles of filaments in the layers" is permitted. Further, "This embodiment can be employed in a fluid conveyance pipe having bending loads in addition to internal pressure loads and in structural members having bending and axial stiffness requirements". Policelli suggests that the fiber angles can be selected in a range between 5.degree. and 75.degree. as measured from the axis.
U.S. Pat. No. 4,556,340 to Morton discloses the use of an externally mounted strip on a flexible pipe. The strip may be of any material having large axial stiffness in tension and low axial stiffness in compression. The strip provides "brased bending" (or preferred axis bending).
U.S. Pat. No. 4,728,224 to Salama discloses a composite mooring tendon on interspersed layers of carbon fibers and aramid fibers, the fibers being axial or low angle helical wrap. A layer of 90.degree. wrap fibers can be provided as an external jacket.
U.S. Pat. No. 4,336,415 to Walling shows a composite flexible tubing assembly for wellbore use with provisions for conveying production fluids and electrical power conduits.
U.S. Pat. No. 3,604,461 to Matthews is concerned with a multiple layer high pressure hose which can slip at the core, i.e. on the inner liner, but be tightly bound at the periphery. The layers of strands are cross plied to one another and are arranged to prevent the migration of the bonding agent through the layers to ultimately prevent the liner from being attached to the outer fibrous layers. The structure of strand layers is thus arranged to move relative to the liner or inner tube of the hose.
U.S. Pat. No. 3,856,052 to Feucht shows a flexible hose with reinforced portions in the sidewalls and parallel to the longitudinal axis to provide longitudinal tensile strength. This makes the hose easily collapsible through one axis and capable of being easily reeled upon itself.
U.S. Pat. No. 646,887 to Stowe et al. has a conductor(s) incorporated in one side-wall of a hose.
For the most part prior-art tubular structures that are designed for being spooled and also for transporting fluids, are made as a hose whether they are called a hose or not. For example the Feucht structure in U.S. Pat. No. 3,856,052 has longitudinal reinforcement in the side walls to permit a flexible hose to collapse preferentially in one plane; however the structure is a classic hose with vulcanized polyester cord plies which are not capable of carrying compression loads or high external pressure loads. Hoses typically use an elastomer such as rubber to hold fibers together but do not use a high modulus plastic binder such as epoxy. Hoses are designed to bend and carry internal pressure but are not normally subjected to external pressure or high axial compression or tension loads. For an elastomeric type material such as used in hoses the elongation at break is so high (typically greater than 400 percent) and the stress-strain response so highly nonlinear; it is common practice to define a modulus corresponding to a specified elongation. The modulus for an elastomeric material corresponding to 200 percent elongation typically ranges from 300 psi to 2000 psi. The modulus of elasticity for typical plastic matrix material used in a composite tube is from 100,000 psi to 500,000 psi or greater, with representative strains to failure of from 2 percent to 10 percent. This large difference in modulus and strain to failure between rubber and plastics and thus between hoses and composite tubes is what permits a hose to be easily collapsed to an essentially flat condition under relatively low external pressure and eliminates the capability to carry high axial tension or compression loads while the higher modulus characteristic of the plastic matrix material used in a composite tube is sufficiently stiff to transfer loads into the fibers and thus resist high external pressure and axial tension and compression without collapse. The procedure to construct a composite tube to resist high external pressure and compressive loads involves using complex composite mechanics engineering principles to ensure that the tube has sufficient strength. It has not been previously considered feasible to build a truly composite tube capable of being bent to a relatively small diameter, and be capable of carrying internal pressure and high tension and compression loads in combination with high external pressure requirements. Specifically a hose will not sustain high compression and external pressure loads.
Matthews U.S. Pat. No. 3,604,461 on the other hand shows a tubular member for carrying relatively high internal pressures but is still a hose made up of layers of fiber strands that are joined to adjacent strand turns. The strands and layers are attached by an adhesive but the adhesive does not impregnate the fibers so that the adhesive does not penetrate to the plastic core. Matthews teaches to position the strands so that they do not shift relative to one another but flexibility would be impaired if the adhesive surrounded the fibers and layers. If the fibers are not individually encapsulated by the matrix, the fibers do not have the capability to carry significant compressive or external pressure loads, a characteristic which is essential to performance of the invention in coiled tubing applications which requires bending as well as pushing on the tube and the ability to withstand external pressure. In this regard Matthew teaches that the liner must not be bonded to the hose. The Matthews structure would permit high external pressures to penetrate to the liner plastic core and collapse it.
The references which show the use of electrical conductors in a hose do so by making them inclusions in the walls of the hose and not as integral structural members which function to strengthen the hose or provide it with particular structural qualities. The electrical and optical conductors as positioned at or near the minimum moment of inertia to isolate them from significant bending strains imposed during bending were they positioned elsewhere. The prior art hoses with conductors are not particularly concerned with protection of the conductors from bending forces imposed by spooling the hose since the hose can be flattened to reduce the effect of bending. In the current invention, the tube must stay generally circular and be able to resist collapsing as high bending strains are imposed during spooling and as the tube buckles as it is pushed into the wellbore.
The Invention
In accordance with the invention, composite tubing is provided for borehole operations such as for use in well logging and workover operations in oil wells. The tubing which is preferably spoolable comprises a composite tubular member having an outer composite structure containing high strength and stiffness fibers embedded in a resin material such as epoxy. The fibers are oriented to resist internal and external pressure and provide low bending stiffness. Two inner areas within the outer structure are located near the neutral axis of the composite tubular member and positioned in diametrically opposite walls of the outer composite structure to provide selective reinforcement within these opposite walls. The inner areas define a minor moment of inertia of bending extending diametrically through the inner areas and a major moment of inertia of bending generally orthogonal to the minor moment of inertia, the inner areas including composite structures oriented along the axis of the tube to provide high axial stiffness and strength to the outer tubular member such that the composite tubular member has significantly greater bending stiffness about the major axis as compared to the bending stiffness about the minor axis thereby providing a preferred direction of bending for the composite tubular member when spooled and unspooled. The arrangement of fibers or other structural material in said inner areas is oriented to provide high axial stiffness, high compressive and tensile strength and low bending stiffness and to resist shear stress. Fibers of high strength and modulus are embedded and bonded into a matrix that keeps the fibers in position, acts as a load transfer medium and protects the fibers from environmental damages. The plastic binder in which the fibers are embedded to form the matrix will have a modulus of elasticity (hereinafter modulus) that exceeds 100,000 psi. The liner also serves as a pressure containment member to resist leakage of internal fluids within the tubing. The liner in one configuration has a circular bore which is required in some downhole operations. In another configuration the liner is made concave to accommodate selective reinforcement members in the sidewall of the outer structure which in turn permits the outer surface of the composite coiled tubing to be substantially circular.
The outer composite structure is comprised of layers or plies of oriented fibers embedded as described in a matrix, which outer composite structure is bonded to a liner so that the liner is integrally attached to the outer composite structure to prevent external pressures from being applied directly to the outer surface of the liner, to thereby prevent collapse of the liner. Additionally, energy conductors including electrical wiring or fiber optics may be made integral to the tubular member, and also arranged to augment the desired physical characteristics of the tubing. Energy conductors commonly have low strain capability and thus can be damaged easily by large deformation such as imposed by bending. Placement of the energy conductors within the inner area ensure that the energy conductors are not exposed to significant bending strains.